The present invention relates to a reading lens system, and, more particularly, to a small-size, high-performance, large-aperture and wide-angle reading lens system suitable for use in a facsimile machine, an image scanner, a micro film machine, etc.
In order to attain a high resolution of the order of 7 to 10 .mu.m on the image side, lenses used in facsimile machine, image scanners, micro film machines, etc., are required to provide a high contrast ratio at high spatial frequencies. In addition, such lenses must have an aperture efficiency close to 100% in order to minimize the decrease in the quantity of marginal light. It is also necessary to compensate various aberrations to a satisfactorily small level.
In order to meet all of these requirements, a Gauss-type lens system of the four-unit six-element lens configuration has conventionally been employed in these applications. However, the Gauss-type lens system has a large amount of comatic flare at middle-angle positions and can achieve an aperture ratio only of the order of 1:4. The demand for reducing the size of the overall lens system is always present, and there is also a strong need to shorten the focal length to accommodate wider viewing angles. However, the Gauss-type lens system has a sudden increase in field curvature for half viewing angles exceeding about 20.degree..
A reading lens system could be designed as a wide-angle lens system in which a negative lens is disposed as the outer member of the system. This type of lens system would have a wide viewing angle but, on the other hand, the length and the outside diameter of the lens system become unduly great and the aperture ratio that can be achieved is still only the order of 1:4.
Cost reduction is another important requirement for modern lens system employed in facsimile machines, image scanners, micro film machines, etc., and hence efforts to increase the aperture ratio and to improve the lens performance cannot be made without considering the need to reduce the number of lens components to the necessary minimum level.